Quick Answer
Vaillant boilers show an F-code when something has locked the appliance out for safety. F.22 means low water pressure, and it’s usually the one you can fix yourself with the filling loop. F.28 means ignition failure.
F.75 means the pressure sensor didn’t detect the pump running, and on real jobs across West and North West London that’s almost always sludge blocking the sensor, not a dead pump. Some codes are safe to check and clear yourself. Others need a Gas Safe registered engineer.
Repeatedly resetting the wrong one is not safe. This guide covers what the common Vaillant codes mean, what we check first on a real job, and where the line sits. See our boiler repairs service if yours needs a Gas Safe engineer.
Vaillant is one of the most common boilers we work on across West and North West London, and the F-codes are consistent across most of the ecoTEC range. Here’s what each common code means, what we check first, and where the manual and real jobs sometimes disagree.
Key Takeaways
- F.75 is the most common Vaillant fault we see. Almost always sludge blocking the pressure sensor, not a dead pump.
- F.22 is usually safe to fix yourself, via the two grey filling loop valves. F.28 is not a DIY job.
- The manual calls F.61 and F.62 a gas valve fault. On real jobs, it’s more often a failing main PCB.
- Never reuse the G10 burner door seal on a service. It’s cheap. A leak from a worn one isn’t.
- Most third-party smart thermostats aren’t wired via Vaillant’s eBUS, and that wrecks the boiler’s efficiency without anyone noticing.
Common Vaillant fault codes and what they mean
F.20
The boiler has overheated and its safety cutout has tripped
No
F.22
Low water pressure
Yes, top up via the filling loop
F.28
Ignition failure. No flame detected after ignition attempts
No
F.29
Flame lost during operation, after it had lit
No
F.61
Gas valve fault, per the manual
No
F.62
Gas valve closing fault, per the manual
No
F.75
Pressure sensor didn’t detect a change when the pump started
No
Exact wording and behaviour can vary a little by model within the ecoTEC range, so always check the code against your own boiler’s display and manual before acting on it.
The real diagnostic order
Always check the pressure gauge first, whatever code is showing. If it’s F.22, that’s just low pressure. It is often the whole story. For F.28, check the gas hob next. If the hob lights, gas is reaching the property. That rules out a supply issue immediately before anything else gets touched.
What does F.75 mean, and why is it so common?
F.75 is the single most common Vaillant fault we see on real jobs. The code itself means the boiler’s pressure sensor didn’t detect a change in pressure when the pump started. Most guides stop there and blame the pump. On the tools, it’s almost always sludge blocking the pressure sensor rather than a dead pump. A clean, protected system rarely throws F.75. An older, uninhibited one does it repeatedly.
Symptoms that tell you which F.22 you’ve got
Not every F.22 is identical. If the pressure drops to F.22 slowly, over several weeks, that’s usually a small radiator or joint leak somewhere in the system. Small leaks show up slowly. If the gauge shoots up toward 3 bar whenever the heating runs, dumps water outside through the pressure relief valve, then settles back to F.22 once the system cools, the expansion vessel has failed. Both end up showing the same code. They need different fixes.
What’s safe to check yourself
You can safely top up the pressure using the two grey filling loop valves underneath the boiler to clear an F.22, provided it isn’t dropping fast or returning within days. That’s normal homeowner territory. It’s the one Vaillant fault most people can sort without an engineer.
What you must never do is take the white casing off to clean the electrodes for an F.28. Ignition components sit next to gas and combustion parts. That work is illegal without Gas Safe registration, not just inadvisable. If F.22 keeps coming back, or you’re looking at F.28, F.29, F.61, F.62 or F.75, that’s a job for a Gas Safe engineer.
The manual says one thing. The tools often say another.
Vaillant’s manual lists F.61 and F.62 as gas valve faults. That’s the official line. In our experience on real jobs, F.61 and F.62 are more often a failing main PCB than the gas valve itself. It’s still worth an engineer confirming which, since replacing a PCB and replacing a gas valve are different jobs with different parts, but don’t be surprised if the official explanation and what gets replaced don’t quite match. See our guide to boiler repair costs for typical Vaillant replacement parts pricing.
The G10 burner seal trap
Every time the combustion chamber gets opened for a service, the G10 graphite burner door seal comes off. Some engineers reuse it to save a few pounds on the part. We don’t. We never reuse old seals. A worn G10 seal that’s reused rather than replaced risks a combustion gas leak into the room, and that’s not a corner worth cutting for the cost of a genuine Vaillant seal. If you’re booking a Vaillant service, it’s worth asking whether the seal gets replaced as standard.
The smart thermostat mistake
Vaillant boilers talk to compatible controls over eBUS, a proper two-way protocol. It’s rare to find a third-party smart thermostat, a Nest or a Hive, wired that way on a London installation. Most get wired as a basic switched-live relay instead, the kind of connection any boiler would accept. The boiler doesn’t know it’s being asked to modulate. It just runs at maximum output until the room hits temperature, which destroys the high-efficiency condensing behaviour the boiler was bought for, without anyone realising why. If your smart thermostat isn’t correctly eBUS-linked to a Vaillant boiler, you’re probably not getting the efficiency you paid for.
We work on Vaillant boilers across West and North West London
We see Vaillant ecoTEC boilers regularly across Queen’s Park, Kilburn, Kensal Rise, Maida Vale, Notting Hill, Willesden and Acton. We also cover Harrow, Wembley and Ealing.
Frequently asked questions
How do I reset my Vaillant boiler?
Press and hold the reset button, shown as a flame with a cross through it, for a few seconds. One reset is normally fine after dealing with an obvious cause. Resetting endlessly on a genuine fault like F.28 or F.75 doesn’t fix it, and it isn’t safe. Stop after the second attempt. Call an engineer.
What is the most common Vaillant fault code?
F.75. It means the pressure sensor didn’t detect a change in pressure when the pump started. Most guides blame the pump, but on real jobs across West and North West London it’s almost always sludge blocking the sensor. An older, uninhibited system throws it repeatedly, while a clean, protected one rarely does. Either way, it’s a job for a Gas Safe engineer.
Can I fix a Vaillant F.22 fault myself?
Yes, usually. Top up the pressure to around 1.5 bar using the two grey filling loop valves underneath the boiler. If it keeps dropping or comes straight back, that points to a leak. If the gauge shoots toward 3 bar and dumps water outside, the expansion vessel has failed. Both need an engineer.
Why does my Vaillant boiler keep cutting out?
A few different faults can look like this. An overheat cutout (F.20), low pressure (F.22), or a loss of flame during operation (F.29) all stop the boiler mid-run. The code on the display tells you which, so note it down before you call. Check the pressure gauge first, since low pressure is the easiest to fix.
What does Vaillant F.28 mean, and is it safe to fix myself?
F.28 means the boiler tried to light and detected no flame after its ignition attempts. Check the pressure gauge, then see whether your gas hob lights, which tells you gas is reaching the property. Beyond that, leave it. Taking the casing off to clean the electrodes is illegal without Gas Safe registration, because ignition parts sit next to gas and combustion components.
How much does it cost to have a Vaillant boiler fault diagnosed?
Our weekday daytime diagnostic visit is £120 (Monday to Friday, 8am to 6pm) across West and North West London. Saturday and evening visits cost more, and we confirm the exact price when you book. Once the engineer has found the fault, we give you a fixed upfront quote for the repair before any work starts, so there are no surprises on the bill.
Is it safe to keep using my Vaillant boiler with a fault code showing?
It depends on the code. After an obvious cause like low pressure, one reset is normally fine. Resetting repeatedly on a genuine fault like F.28 or F.75 doesn’t fix it and isn’t safe, so stop after the second attempt and call a Gas Safe engineer. If you ever smell gas, leave the property and call the National Gas Emergency Service on 0800 111 999 first.
Need a Gas Safe engineer for your Vaillant boiler?
Tell us the fault code and what it’s doing, and we’ll get an engineer to you with a clear fixed price agreed before any work starts.
- WhatsApp us and book in 60 seconds
- Call 020 8051 2124. Our line is answered 24/7 and we’ll get your details to an engineer
- See the service: boiler repairs · Homeowner Plan
We’re Gas Safe registered (657972) and cover West and North West London. Heating that just works, no drama.
Vaillant Fault Code Showing in West London?
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